variable costs are costs that vary with the level of _____ and are generally assumed to be proportional to the level of _____. Total variable costs (VC) equal ____ multiplied by the ______. This relationship is summarized algebraically as : VC = _________. Costs which do not change over a specified time interval and must be paid regardless of production levels are called ______. Total costs (TC) for a given lelvel of output in a given period are defined as the sum of _______ and ________. Algebraically, TC = _________